PROJECT 1: ABSTRACT. A key problem in understanding and eventually treating Alzheimer's disease (AD) is our incomplete understanding of the role of genetic risk factors in late-onset, sporadic AD (SAD). While there is no clear single genetic lesion that causes SAD, the observed high heritability suggests that individual genetic background plays a significant role. Here we propose unique applications of human induced pluripotent stem cell (hIPSC) technology to dissect how individual genetic background and identified risk factors predispose to SAD biochemical phenotypes in human neurons and to link that information to clinical data on individual patients and to post-mortem pathology. We are basing our work on our recent finding using hIPSC technology that tested the hypothesis that R haplotypes cause general reduction of SORL1 expression leading to increased amyloid beta (A) peptides and consequent risk of developing SAD [3, 7- 10]. Using hIPSC technology we found that R haplotypes impair a signaling input to the SORL1 gene. Specifically, P haplotypes respond to BDNF by inducing SORL1 expression, while R haplotypes do not. Basal expression levels show no correlation with R or P haplotypes. Thus, the SORL1 genetic contribution to SAD may be caused by complex regulatory variation in living human neurons. We now propose to test our working model for how SORL1 haplotypes contribute to SAD neuronal phenotypes and thus SAD risk in humans in vitro, and in vivo in human patients. We propose to test: 1) the hypothesis that BDNF-induced SORL1 expression modulates amyloidogenic processing of APP and downstream SAD-associated biochemical changes; and 2) the hypothesis that effects of SORL1 haplotype on neuronal phenotypes in vitro are mirrored in clinical data on SAD patients, specifically the relative amounts of BDNF and SORL1 proteins in cerebrospinal fluid (CSF) and post-mortem neuropathological phenotypes. In a related goal, we will investigate whether purified neurons made from our collection of patient hIPSC lines correlates with clinical behavior of individual patients. At present, we have too few hIPSC lines to definitively establish the degree of correlation rigorously, but given the existence of the lines, we will begin to collect comparative data as a way of contributing to future studies. Together, these experiments will provide new data about the details of SORL1 variant contributions to SAD phenotypes in human neurons with differing genetic backgrounds and potentially lead to new pathways for drug discovery and stratification of clinical trials based on genetic background. Our specific aims are to: 1. Test the hypothesis that the reduced BDNF induction response of purified human neurons with SORL1 risk variant haplotypes enhances SORL1- dependent downstream biochemical SAD phenotypes. 2. To test the hypothesis that the genetic status of patients at the SORL1 locus has a significant influence on clinical phenotypic markers measured in CSF or by post-mortem pathology.